Abstract: Wellbore instability is an important constraint for improving the drilling speed of ultra-deep wells. Engineering
geological environment including high confine pressure，high temperature and high pore pressure is the key cause of borehole
instability. Borehole stability evaluation model is derived from stress condition，temperature perturbation and pore pressure
around well under the deep geological conditions. The effect of stress state，formation temperature and pore pressure
on borehole stability is quantitatively analyzed base on the model. The following conclusions have been drawn：（1）in-situ
stress has significant impact on borehole stability，and the increase of difference between three principal stresses will aggravate
wellbore instability.（2）The increase of temperature will increase the possibility of Shear failure，but weaken the tensile
instability.（3）High pore pressure will aggravate wellbore instability.

Key words: wellbore stability, engineering geology, In-situ stress, pore pressure, temperature, mechanical evaluation